1. Field of the Invention
The field of the invention is that of devices for regenerating optical signals. It applies more particularly to high-throughput long-distance systems for transmission by optical fibres of digital data. The throughputs transmitted by this type of link are typically several tens of gigabits per second and can exceed a terabit per second.
These long-distance transmissions can be performed, for example, by means of underwater cables.
2. Description of the Prior Art
The digital optical signals S which propagate inside an optical fibre consist of temporal pulses conventionally representing 1 or 0 logic levels. By way of example, FIG. 1 represents as a function of time, the variation of amplitude A of an initial signal S1 comprising a succession of 1 or 0 logic levels. These signals generally have a format of RZ-OOK type, the acronym standing for Return-to-Zero On-Off Keying. While propagating, this signal S1 necessarily experiences attenuation and degradation of its envelope and of its signal/noise ratio as indicated in FIG. 2 which represents the signal S2 after propagation in an optical fibre.
To limit this phenomenon, a first possible solution consists in carrying out management of the optical dispersion occurring along the line. Powerful emission sources are used to this end and the distance separating two consecutive optical amplifiers is limited by taking account of the chromatic dispersion of the fibres used. These amplifiers are, for example, of the EDFA type, the acronym signifying: Erbium Doped Fibre Amplifier.
So that this signal can be utilized correctly, a second solution consists in regenerating it periodically. Very conventionally, regeneration comprises 3 processes which are called:                Resynchronization: The signal experiences phase noise. The pulse then drifts temporally. This phenomenon is also called “jitter”. Consequently, resynchronization consists in resynchronizing the pulses with respect to a reference clock.        Reamplification: The attenuation of an optical fibre typically equals 0.2 dB/km. Over very long distances, greater than 1000 kilometres, it then becomes necessary to periodically reamplify the signal so that it remains utilizable.        Reshaping: The signal also experiences amplitude noise. The low and high parts of the pulse rectangles are noisy as may be seen in FIG. 2. It is therefore necessary to eliminate or to attenuate this noise.        
When these 3 processes are implemented, one speaks of 3R regeneration. It is possible to demonstrate that resynchronization is not fundamental for certain applications. It is thus possible to produce a transoceanic link of more than 6000 kilometres without resynchronization. One then speaks of 2R regeneration (Reamplification and Reshaping).
To carry out this 2R regeneration, a possible method consists in carrying out a first transduction of the initial optical signal into an electronic signal, then in processing the electronic signal thus obtained, lastly in carrying out a second transduction of the processed signal into a final optical signal. When the signal is wavelength multiplexed, also called a WDM signal, the acronym signifying “Wavelength Division Multiplex”, it is necessary to carry out regeneration on the whole set of elementary channels making up the WDM signal. This method then exhibits the main drawbacks of being expensive and complex, especially if the number of optical channels to be processed is significant and of course, the numerous opto-electronic transductions required decrease the reliability of the device.
Also, so-called all-optical procedures have been proposed. Generally, they rely on the use of structures with saturable absorbent.
The principle of optical regeneration with saturable absorbent is depicted in FIGS. 3, 4 and 5.
FIG. 3 presents a sectional view of an optical structure 1 with saturable absorbent. This structure 1 comprises essentially:                An active layer 2 consisting of a material with saturable absorbent;        Two reflecting mirrors 3 and 4 disposed on either side of the active layer 2;        So-called phase layers 5 and 6 disposed between the active layer 2 and the reflecting mirrors 3 and 4.        
The optical structure is transferred onto a substrate 7.
The structure generally operates by reflection of light. In FIG. 3, the course of the light path through the structure is symbolized by straight barred arrows.
As illustrated in FIG. 4, the material with saturable absorbent is a material whose absorption coefficient α varies with the luminous power received. Thus, low powers are weakly absorbed while strongly/higher optical powers are weakly absorbed. The dynamic swing of the phenomenon being very significant, the variation of the absorption coefficient and the optical power are represented on logarithmic scales, the optical power conventionally being represented in dBm. 0 dBm corresponds to a power of a mW and 30 dBm to a power of 1 watt.
Generally, the active layer 2 of the absorbent is made either of ternary material, in particular of InGaAs or of AlGaAs, or of quaternary material.
The reflecting mirrors 3 and 4 make it possible to generate, inside the active layer, multiple reflections of the optical signal, thus increasing the optical path inside the active layer and multiplying its absorption effectiveness. In order that the multiple reflections are all in phase, phase layers 5 and 6 make it possible to adapt the optical length of the cavity situated between the mirrors 3 and 4.
It was seen that the noisy signal S2 is composed of deformed rectangular light pulses. After reflection by the whole of the structure, the signal S2 has become the signal S3, the spurious noise of the low parts of the rectangles corresponding to the 0 logic levels has been in large part absorbed as illustrated in FIG. 5. The signal-to-noise ratio of the pulses is thus increased.
However, this procedure exhibits a drawback. As may be seen in FIG. 5, the spurious noise of the high parts of the temporal rectangles of the signal corresponding to the 1 logic levels is not attenuated. One thus speaks of 1.5R regeneration with reference to this phenomenon insofar as only the 0s of the signal are regenerated. Thus, if we desire full 2R regeneration, it is necessary to supplement the devices comprising structures with saturable absorbent with other optical devices making it possible to regenerate the 1 logic levels of the signal. These devices generally comprise compression fibres and/or filters.